The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
BS base station
BW bandwidth
CP cyclic prefix
DL downlink (eNB towards UE)
eNB E-UTRAN Node B (evolved Node B)
EPC evolved packet core
E-UTRAN evolved UTRAN (LTE)
FDMA frequency division multiple access
ID identity/identification
LTE long term evolution of UTRAN (E-UTRAN)
MAC medium access control (layer 2, L2)
MBSFN multicast/broadcast single frequency network
MM/MME mobility management/mobility management entity
Node B base station
OFDM orthogonal frequency division multiplexing
OFDMA orthogonal frequency division multiple access
O&M operations and maintenance
PDCP packet data convergence protocol
PHY physical (layer 1, L1)
PSS primary synchronization signal
QPSK quadrature phase-shift keying
RLC radio link control
RRC radio resource control
RRM radio resource management
RS reference signal
RSRP reference signal received power
RSRQ reference signal received quality
S-GW serving gateway
SC-FDMA single carrier, frequency division multiple access
SSS secondary synchronization signal
TDD time division duplex
TS technical specification
UE user equipment, such as a mobile station or mobile terminal
UL uplink (UE towards eNB)
UTRAN universal terrestrial radio access network
A communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. As presently specified the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest is 3GPP TS 36.300, V8.6.0 (2008-09), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8),” incorporated by reference herein in its entirety.
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300 V8.6.0, and shows the overall architecture of the E-UTRAN system 2. The E-UTRAN system 2 includes eNBs 3, providing the E-UTRAN user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE (not shown). The eNBs 3 are interconnected with each other by means of an X2 interface. The eNBs 3 are also connected by means of an S1 interface to an EPC, more specifically to a MME by means of a S1 MME interface and to a S-GW by means of a S1U interface (MME/S-GW 4). The S1 interface supports a many-to-many relationship between MMEs/S-GWs and eNBs.
The eNB hosts the following functions:                functions for RRM: RRC, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both UL and DL (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards the EPC (MME/S-GW);        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
To reduce or avoid reception problems for multi-path radio signals, a symbol (e.g., an OFDM symbol) may be extended by a CP. At the transmitter, the last part of an OFDM symbol is inserted at the beginning of the same OFDM symbol. At the receiver, after synchronization the CP of the OFDM symbol is ignored. If two signals are received due to multi-path issues, the switch between two consecutive OFDM symbols in the delayed signal should occur within the CP and thus should not cause a problem (e.g., interference). While a CP may slightly reduce the effective throughput (i.e., due to repetition of data), use of a CP provides a more robust signal that is more resistant to data errors, such as those caused by multi-path reception.
In addition to a “normal” CP, E-UTRAN also provides for an “extended” CP having a greater length/duration. The extended CP is defined for large cell scenarios with higher delay spread and MBMS transmission. For example, as was specified for Δf=15 kHz (specified in TS 36.211 V8.2.0, see below for complete citation), the length of the normal CP is 160 samples for the first symbol and 144 samples for other symbols within a 0.5 ms slot (approximately 5 microseconds), while the extended CP is 512 samples long (approximately 17 microseconds).
The length of the CP is detected blindly by the UE (e.g., from the time distance between the PSS and the SSS). However, due to timing issues with neighbor cells, the CP length may be detected incorrectly by the UE during cell search. Two error cases will be considered. For these error cases, assume that the UE is searching/measuring a first cell (“the searched/measured cell”) in the presence of a second cell (“the neighbor cell”). Furthermore, as specified in the error cases, assume that the searched/measured cell and the neighbor cell use two different CP lengths, for example, the normal CP length and the extended CP length (i.e., a CP having a different length than the normal CP).
(i) The searched/measured cell has a normal CP while the neighbor cell has an extended CP. The neighbor cell uses the same PSS code as the searched/measured cell and the reception timing of the searched/measured cell's PSS is based on the extended CP length instead of the normal CP length. FIG. 3 illustrates the first error case for a FDD system. FIG. 4 shows the first error case for a TDD system.
(ii) The searched/measured cell has an extended CP while the neighbor cell has a normal CP. The neighbor cell uses the same PSS code as the searched/measured cell and the reception timing of the searched/measured cell's PSS is based on the normal CP length instead of the extended CP length. FIG. 5 illustrates the second error case for a FDD system. FIG. 6 shows the second error case for a TDD system.
In both of the above cases, the UE will detect PSS timing according to the neighbor cell and SSS timing and cell ID according to the searched/measured cell. That is, the UE will detect the first cell's ID but the second cell's timing.
In view of the above-identified error cases, it is desirable to employ a mechanism (e.g., in the UE) to detect the CP length or avoid CP length misdetection. Such a detection/verification mechanism is typically based on the DL RS and generally includes a step for calculating correlation between the received DL RS and the DL RS replica. To provide a reliable verification, it is desirable that the RS correlation hypothesis results calculated with the correct timing are very different (e.g., as different as possible) from the RS correlation hypothesis results for the incorrect timing.
The previously-specified DL RS mapping and scrambling initialization for E-UTRAN implies that for a given cell ID and a given subframe number:
(a) All RS sub-carriers will use the same cell-specific frequency shift regardless of the CP length.
(b) The RS in the first OFDM symbol of each sub-frame or slot will use the same Gold (QPSK) scrambling sequence in both cells (because the RS scrambling sequence generator is initialized in the same manner with {OFDM symbol number, subframe number, cell ID} or with {OFDM symbol number, slot number, cell ID}).
Reference with regard to the above may be made to TS 36.211 V8.2.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” March 2008. Reference may also be made to this document concerning the specifications for the PSS, SSS, DL RS sequences and mapping.
The above implications suggest that a considerable portion of the DL RS signal (in the first OFDM symbol of each subframe or each slot) will be very similar for both the normal CP and the extended CP. This will make CP length verification more complex and/or less reliable. It should also be noted that the RS in the first OFDM symbol may constitute a large portion of available DL RS symbols and thus be more important for UE measurements and auxiliary functions, for example, in the presence of mixed MBSFN carriers and/or TDD carriers (due to a shorter DL unicast portion of subframes) or for inter-frequency measurements (due to short measurement gaps).